# C++ named requirements: LiteralType (since C++11)

From cppreference.com
< cpp‎ | named req

Specifies that a type is a literal type. Literal types are the types of `constexpr` variables and they can be constructed, manipulated, and returned from `constexpr` functions.

Note: the standard doesn't define a named requirement with this name. This is a type category defined by the core language. It is included here as a named requirement only for consistency.

### Requirements

A literal type is any of the following:

possibly cv-qualified void (so that constexpr functions can return void); | (since C++14)

- scalar type;
- reference type;
- an array of literal type;
- possibly cv-qualified class type that has all of the following properties:

  - has a trivial(until C++20)constexpr(since C++20) destructor,
  - all of its non-static non-variant data members and base classes are of non-volatile literal types, and
  - is one of

a lambda type, | (since C++17)

    - an aggregate union type that
      - has no variant members, or
      - has at least one variant member of non-volatile literal type,
    - a non-union aggregate type, and each of its anonymous union members
      - has no variant members, or
      - has at least one variant member of non-volatile literal type,
    - a type with at least one constexpr (possibly template) constructor that is not a copy or move constructor.

### Notes

A type can be literal even if all of its constexpr constructors are deleted, inaccessible, or cannot participate in overload resolution.

    struct A { constexpr A(int) = delete; char c; }; // A is a literal type
    constexpr A v = std::bit_cast<A>('0'); // OK in C++20
                                           // v has literal type and thus can be constexpr

### Example

Literal type that extends string literals:

Run this code

    #include <cstddef>
    #include <iostream>
    #include <stdexcept>

    class conststr // conststr is a literal type
    {
        const char* p;
        std::size_t sz;
    public:
        template<std::size_t N>
        constexpr conststr(const char(&a)[N]) : p(a), sz(N - 1) {}

        constexpr char operator[](std::size_t n) const
        {
            return n < sz ? p[n] : throw std::out_of_range("");
        }

        constexpr std::size_t size() const { return sz; }
    };

    constexpr std::size_t count_lower(conststr s)
    {
        std::size_t c{};
        for (std::size_t n{}; n != s.size(); ++n)
            if ('a' <= s[n] && s[n] <= 'z')
                ++c;
        return c;
    }

    // An output function that requires a compile-time constant N, for testing
    template<int N>
    struct constN
    {
        constN() { std::cout << N << '\n'; }
    };

    int main()
    {
        std::cout << "The number of lowercase letters in \"Hello, world!\" is ";
        constN<count_lower("Hello, world!")>(); // the string literal is implicitly
                                                // converted to conststr
    }

Output:

    The number of lowercase letters in "Hello, world!" is 9

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
CWG 1453 | C++11 | a literal class could have volatile data members | not allowed
CWG 1951 | C++11 C++14 | it was unclear whether cv-qualified void (C++14) and class types (C++11) are literal types | they are
CWG 2096 | C++11 | for a union type to be literal, all its non- static data members must be literal | only one non-static data member needs to be
CWG 2598 | C++11 | for a union type to be literal, it must have at least one non-static data member | it can have no non- static data member

### See also

is_literal_type (C++11)(deprecated in C++17)(removed in C++20) | checks if a type is a literal type (class template)

Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/named_req/LiteralType&oldid=177799"